In recent years, regeneration of used papers attracts more attention in the perspective of environmental protection, resource conservation, and garbage reduction. Today, used papers of newspapers, magazine papers, corrugated papers, etc. are mainly used as sources of used papers for raw materials of these regenerated papers. Recently, an activity for collection of waste office papers out of offices by the whole building is also found, thus the regeneration of the used papers is becoming more important technology.
Manufacturing recycled papers generates a large amount of paper sludge, such as deinked froth discharged from used paper treatment processes, wastewater and dehydration sludge discharged from respective paper manufacturing processes etc.
The paper sludge contains many inorganic substances, such as fillers applied to various papers, and pigments for coated layers of papers.
Conventionally, this paper sludge is burnt for volume-reduction, or landfill disposal. However, since the paper sludge contains a large amount of inorganic substances, burning of the paper sludge leaves a large amount of ash (inorganic substance), leading to a low effect in volume reduction. For this reason, the use of ash is intended as raw material of cement, soil conditioner and so on. However, in such use, a large amount of ash cannot be used, because the ash is merely served as an auxiliary agent. After all, most of ash is to be used for landfill disposal.
There can be a method for effective use of the ash as an internal filler of a paper as it is without any treatment. However, the ash cannot be used properly as the internal filler as it is without any treatment due to its low brightness.
Therefore, Patent Document 1 proposes a method, where before the use of ash, the ash is re-burnt for improving its brightness.
In the method described in Patent Document 1, however, when the ash is re-burnt, it is necessary to set the re-burning temperature at 500 to 900° C. for perfect burning of the unburnt carbon, which means that the brightness of the ash is improved only to about 50%. Thus, it has been known that it is not suitable for the ash to be used as filler in a paper. Furthermore, it has been found that when the re-burning temperature is set to be 900° C. or higher, the ash (inorganic substance) is sintered, molten and extremely hardened.
In addition, since the re-burnt ash has a very high hardness, when it is used as filler, abrasion of paper making wires advances quickly, thereby its lifetime is shortened very much, resulting in impossibility of actual application.
Against such defect, there can be a countermeasure such as decreasing the particle diameter of the re-burnt ash by pulverizing it for abrasion reduction and improvement in smoothness. Use of the ash as an internal filler, however, extremely decreases the retention in the paper making and raises the energy cost for grinding due to an extraordinary high hardness of the ash itself.
Patent Document 2 proposes a method of regeneration of inorganic substances contained in paper sludge as a pigment for paper making by supplying the paper sludge in a reactor containing an oxygen-containing gas introduced thereinto, and by oxidizing for 0.25 to 5 hours under a condition of 250 to 300° C. and about 3000 psig.
However, due to wet type air-oxidation treatment of paper sludge, this method may only provide insufficient removal of organic substances, a low brightness of the obtained pigment, and uneven particle diameters, and therefore the pigment is unsuitable for use as a filler and a pigment for paper making, leading to a complicated reaction operation and higher cost.
On the other hand, Patent Document 3 proposes a method, wherein after obtaining paper sludge charcoal by fumigation combustion of paper sludge, clay is formed as a raw material for paper making by further burning of the obtained charcoal by kiln. However, this method has a large disadvantage of failing in effective extraction of energy from the paper sludge due to fumigation combustion of the paper sludge, conversely needing input energy. Furthermore, the method has a large problem that the formed clay also has uneven large particle diameters, and cannot be used as a pigment for coating.
Moreover, as in Patent Document 4, a method of treatment by continuous drying, carbonization, and calcination of sludge formed in waste water treatment in a rotary kiln is also known. In this method, processes of granulation and molding before calcination are performed for obtaining uniform calcination. In continuous drying, carbonization, and calcination in the rotary kiln under a condition of 40 to 60% of solid concentration (in other words, 60 to 40% of moisture percentage) as described in an embodiment, compulsory treatment of the sludge particle advances by the rotation of the kiln irrespective of a situation of dried condition and carbonized condition. Accordingly, insufficient drying makes a large unburnt amount left in the particles, and causes imperfect calcination, and consequent reduction of the brightness. Conversely, excessive drying causes excess calcination, while allowing perfect calcination, and a high hardness of the obtained regenerated particles. As a result use of this regenerated particle disadvantageously causes wire abrasion in paper machines, and abrasion of a cutter blade in cutting of papers.
The biggest problem described in the preceding Patent Documents 1 to 4 in the case of use of the paper sludge as a raw material is in a mixed condition of a variety of paper sludge including paper sludge flown out through wires in a paper making process, paper sludge collected from the wastewater containing solid contents generated in a washing process in a pulping process, a solid content separated and collected with a solid content separation apparatus using precipitation or surfacing in a waste water treatment process, paper sludge after removal of mixed foreign matter in a used paper treatment process, etc.
In this situation, for example, the paper sludge flown out through wires in the paper making process contains intermingled paper strength additives etc., and therefore may have consequent variation in quality due to variation of paper milling materials in the paper making process.
The wastewater sludge may contain intermingled flocculants, and furthermore may have large variation in quality based on paper made products of the whole plant, the variation of the amount of production, washing within the process of the production facility, etc.
The paper sludge formed from the washing process in the pulping process may contain many substances that does not allow use of the paper sludge as various fillers and pigments caused by the variation of the chip moisture or the pulp manufacturing conditions, leading to variation in quality. The paper sludge formed from the washing in the pulping process has variation in quality depending on the chip moisture or pulp-manufacturing conditions as well as various substances which cannot be used as fillers or pigments and are mixed into the sludge.
Accordingly, use of all the paper sludge without selection causes large deterioration in quality of the fillers and pigments for paper making, and extremely large variation in quality, resulting in unstable product quality.
That is, each of any regenerated particles conventionally obtained by publicly known methods is merely collected as a paper making particle, and it cannot be used as a regenerated particle for paper making, because its quality is unsuitable and unstable.    [Patent Document 1] Japanese Patent Application Laid-Open No. 11-310732    [Patent Document 2] Japanese Patent Application Publication No. 56-27638    [Patent Document 3] Japanese Patent Application Laid-Open No. 54-14367    [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-176208